The present invention relates generally to data storage systems and, more particularly, to a system and method for measuring absolute clearance between a magnetoresistive transducer and a medium using a thermal response of the MR transducer.
In many applications, it is desirable to ascertain the clearance or spacing between an MR transducer and a data storage disk. By way of example, an unexpected change in head-disk clearance of a particular head is generally indicative of a problem with the head or head assembly. An appreciable decrease in head-disk clearance by one head of a disk drive system relative to other heads may be indicative of a suspect head.
One known method for determining head-disk clearance is referred to as a Harmonic Ratio Flyheight (HRF) clearance test. A HRF testing approach typically requires employment of a dedicated tester which may take several minutes to complete HRF testing of a disk drive. A HRF testing approach, as well as other known flyheight evaluation techniques, require that dedicated tracks of magnetic information be available over which each of the heads must pass with relative high precision in order to obtain an accurate head flying height measurement. Accordingly, such known flyheight measuring approaches require the presence of a previously recorded magnetic pattern on the disk.
Moreover, present HRF measurement techniques are very sensitive to off-track deviations. By way of example, approximately one-half of the time required to perform a HRF measurement during manufacturing involves accurately locating the centerline of the dedicated magnetic test track. Further, HRF testing data becomes highly inaccurate when the low pole frequency of the arm electronics (AE) is near HRF readback frequencies. In such cases, additional attenuation may cause HRF clearance measurement errors in excess of 50 nanometers (nm) for some disk drive systems. Although such attenuation and may be compensated for if the AE low frequency pole can be accurately estimated, known approaches for accurately estimating the AE low frequency pole are problematic for a variety of reasons.
Other known head flyheight evaluation techniques involve the use of the thermal response of an MR head. Reference is made to co-owned U.S. Pat. No. 5,751,510 which discloses techniques concerning the identification, processing, and uses of the thermal signal component of a readback signal, including head flyheight evaluation. U.S. Pat. No. 5,751,510 is hereby incorporated herein by reference in its entirety.
The thermal response of an MR head is particularly useful when evaluating disk topography variations. Techniques that exploit the thermal response of the MR head for purposes of determining head flyheight generally require that the disk be operated at a fixed speed, and generally rely on various calibration or normalization methods so that the thermal response of the MR head is useable. Although such techniques provide for an accurate representation of relative head-disk spacing, such known methods that utilize the thermal response of an MR head are generally not capable of providing an estimate as to absolute head-disk clearance.
There exists a need in the data storage system manufacturing community for an apparatus and method for measuring absolute head-disk clearance at the time of manufacturing and, importantly, during subsequent use in the field. There exists a further need for an apparatus and method for detecting absolute head-disk clearance without the need for dedicated test tracks. There exists yet a further need to provide such an apparatus and method which is suitable for incorporation into existing data storage systems, as well as into new system designs, and one that operates fully autonomously in-situ a data storage system. The present invention is directed to these and other needs.
The present invention is directed to an apparatus and method for measuring absolute clearance between a magnetoresistive transducer and a medium which moves relative to the MR transducer. The medium may be devoid of magnetic information or may include magnetic information. In accordance with the principles of the present invention, a signal is produced using an MR element of the MR transducer, such that the signal varies as a function of clearance between the MR element and the medium. The signal comprises a thermal component representative of a thermal response of the MR element.
The velocity of the medium is reduced relative to the MR transducer. While reducing the medium-transducer velocity, the rate of change of the signal produced by the MR transducer is monitored. Using data associated with the rate of change of the signal during spindown, absolute clearance between the MR transducer and the medium is computed for a nominal medium-transducer velocity, such as a full operational velocity.
Computing the absolute clearance between the MR transducer and the medium may further involve determining a transition velocity at which the rate of change of the signal exceeds a pre-established threshold. The transition velocity is used to compute the absolute clearance between the MR transducer and the medium for a nominal medium-transducer velocity. In one embodiment, for example, a transition velocity represents a medium-transducer velocity at which the rate of change of the signal exceeds the pre-established threshold by about 10% or more.
The transition velocity is representative of a medium-transducer velocity at which the heat transfer behavior of the MR transducer assembly transitions from a first thermal transport mechanism to a second thermal transport mechanism. The transition velocity coincides with a medium-transducer velocity at which appreciable contact occurs between the transducer and the medium.
Computing absolute clearance between the MR transducer and medium may further involve associating a transition velocity, at which the rate of change of the signal exceeds the pre-established threshold, with an absolute clearance value obtained using a clearance profile associated with the MR transducer. For example, the clearance profile may be representative of a relationship between landing or take-off velocity of the MR transducer and associated transducer-medium clearance.
Monitoring the rate of change of the signal produced by the MR element involves performing a plurality of rate of change or slope computations using the signal to establish the pre-established threshold. The signal produced by the MR element may be representative of a resistance of the MR element that varies as a function of transducer-medium clearance. The signal may alternatively be representative of a voltage across the MR element that varies as a function of transducer-medium clearance. During the absolute transducer-medium clearance measurement procedure, the MR element may be biased using a constant current.
According to another embodiment of the present invention, the thermal spindown technique for measuring absolute transducer-medium clearance may also be employed to evaluate lubricity between a slider that supports the MR transducer and the medium. According to this embodiment, the transition velocity at which the rate of change of the signal produced by the MR transducer exceeds a pre-established threshold is determined. The state of lubricity between a slider that supports the MR transducer and the medium may then be determined using the rate of change of the signal for medium-transducer velocities lower than the transition velocity.
The rate of change signal samples acquired for medium-transducer velocities lower than the transition velocity may be characterized in terms of a curve having a given slope. The slope of the curve is related to the state of the temperature profile of slider/MR transducer. If the slope of such a curve is appreciably greater than the slope of previously computed curves for the same slider assembly, the resulting increase in slider/medium frictional heating may be due to insufficient provision of a lubricant between the slider and medium.
According to a further embodiment of the present invention, a surface profile or crown of a slider may be characterized be evaluating the slope of a curve developed using rate of change signal samples acquired for medium-transducer velocities lower than the transition velocity. A transition velocity at which the rate of change of the signal exceeds a pre-established threshold is determined. Since the slope of this curve may also be viewed as a function of the configuration of the slider crown, a surface profile of the slider may be characterized by evaluating changes in the slope of the curve relative to previously computed curves for the same slider.
In accordance with yet another embodiment, information acquired during the thermal spindown procedure may be used to adjust the medium-transducer velocity or a characteristic of a particular slider in order to maintain a desired clearance between the slider and the medium. Slider pre-load or pitch angle, for example, may be adjusted, alone or in combination with a medium-transducer velocity, to maintain a given slider at a desired flyheight relative to the medium.
An apparatus for measuring absolute clearance between a slider and a medium moving relative to the slider according to the principles of the present invention includes an MR transducer comprising an MR element which is supported by the slider. The MR transducer produces a signal that varies as a function of clearance between the MR element and the medium. The signal comprises a thermal component representing a thermal response of the MR element. The apparatus includes a controller that controls a velocity of the medium relative to the slider, and, in particular, reduces the velocity of the medium relative to the slider during the absolute clearance measurement procedure.
A processor, which is coupled to the controller and MR transducer, computes a rate of change of the signal during reduction of the medium-slider velocity and determines a medium-slider threshold velocity at which the rate of change of the signal exceeds a pre-established established threshold. The processor determines, for a nominal medium-slider velocity, absolute clearance between the slider and the medium using the threshold velocity and a clearance profile associated with the slider.
The processor performs a number of rate of change computations using the signal to establish the pre-established threshold. The transition velocity is representative of a medium-slider velocity at which the rate of change of the signal exceeds the pre-established threshold by about 10 percent or more. An apparatus for measuring absolute clearance between a slider and a medium moving relative to the slider according to the principles of the present invention may be incorporated in a data storage system, such as a direct access storage device (DASD), or a test apparatus.
The apparatus may further include a programmable filter, which is coupled to the processor, that filters the rate of signal change computations produced by the processor. In one embodiment, the programmable filter comprises a finite impulse response (FIR) filter. The programmable filter may be programmed to perform a least-squares linear fit or other type of curve fitting technique using the rate of signal change computations produced by the processor.
The processor may also determine a state of lubricity between the slider and the medium using rate of signal change data acquired for medium-slider velocities lower than the transition velocity. The processor may further characterize a surface profile of the slider using rate of signal change data for medium-slider velocities lower than the transition velocity. In another embodiment, the processor cooperates with the controller to adjust the medium-slider velocity or a characteristic of the slider to maintain a desired clearance between the slider and the medium.
The above summary of the present invention is not intended to describe each embodiment or every implementation of the present invention. Advantages and attainments, together with a more complete understanding of the invention, will become apparent and appreciated by referring to the following detailed description and claims taken in conjunction with the accompanying drawings.